**Negative predictive value **of a test/investigation is defined as the proportion of patients with negative results being truly disease free.

#### Calculation

Negative predictive value = true negatives detected / total negative results

(where "total negative results" = true negative + false negative)

###### Bayes' theorem

One can also determine the NPV with an estimate of sensitivity, specificity, and pretest probability (p).

NPV = [(specificity) x (1 - p)] / [specificity x (1 - p) + (1 - sensitivity) x (p)]

#### Practical points

- unlike sensitivity and specificity, NPV is highly dependent on the prevalence of the disease in the target population